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Rhynchonellata - Athyridida - Meristidae
If no rank is listed, the taxon is considered an unranked clade in modern classifications. Ranks may be repeated or presented in the wrong order because authors working on different parts of the classification may disagree about how to rank taxa.
|A. Williams 1962||Exterior.-- Small, subpentagonal, rostrate shells with a deeply convex pedicle valve strongly arched about a broad, rounded, median fold, which is separated from flattened or lowly folded postero-lateral areas by a pair of widely divergent and shallow, rounded sulci, and a gently convex brachial valve depressed by a wide sulcus complementary to the high median zone of the pedicle valve; ventral umbo small but conspicuous, projecting dorsally over the inconspicuous umbo of the brachial valve, foramen circular, suggesting a partial restriction of the delthyrium antero-laterally, interareas absent but the postero-lateral zones of both valves are commonly steeply disposed adjacent to the commissure to form a pair of lanceolate false areas, one on either side of the umbonal region; shell substance impunctate, surface smooth.
Interior of pedicle valve.-- Teeth small, supported by long, slightly divergent, dental lamellae reaching posteriorly to the floor of the valve to define a small pedicle chamber but ankylosed ventrally and anteriorly to a large, obtusely triangular plate disposed more or less parallel with the plane of the posterior commissure but bulging slightly towards the dorsal valve. The sides of the triangular plate are continuous with the postero-lateral zones of the valve floor while the concavely curved anterior edge is free, so that the plate extends across the valve from one side to another to define a postero-ventral chamber.
Interior of brachial valve.-- Hinge-plate divided and including a pair of short subtriangular crura with small sockets sublateral to their bases; primary lamellae continuous with crura and disposed in a pair of single convolutions, parallel with, but not attached to, each other; ribbon broad and sharply puckered into a pair of narrow ridges situated just within the edges of the ribbon; median ridge low apparently not giving support to the hinge-plate.